/*
 * base.h
 * Copyright (C) 2018 wangxiaoshu <2012wxs@gmail.com>
 *
 * Distributed under terms of the MIT license.
 */

#ifndef SRC_BASE_THREAD_POOL_H_
#define SRC_BASE_THREAD_POOL_H_

#include <unistd.h>
#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>
#include <atomic>

namespace xflow {
class ThreadPool {
 public:
    explicit ThreadPool(size_t);
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args)
        -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();
 private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;

    std::atomic_llong tasks_number = {0};

    int queue_max_threads_num = 64;
    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads)
    :   stop(false) {
  for (size_t i = 0; i < threads; ++i)
    workers.emplace_back(
      [this] {
        for (;;) {
          std::function<void()> task;
          {
            std::unique_lock<std::mutex> lock(this->queue_mutex);
            this->condition.wait(lock,
              [this] { return this->stop || !this->tasks.empty(); });
            if (this->stop && this->tasks.empty())
              return;
            task = std::move(this->tasks.front());
            this->tasks.pop();
          }
          tasks_number -= 1;
           task();
        }
      });
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
    -> std::future<typename std::result_of<F(Args...)>::type> {
  using return_type = typename std::result_of<F(Args...)>::type;
  auto task = std::make_shared< std::packaged_task<return_type()> >(
              std::bind(std::forward<F>(f), std::forward<Args>(args)...));
  std::future<return_type> res = task->get_future();
  while (tasks_number >= queue_max_threads_num) usleep(1);
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    if (stop) throw std::runtime_error("enqueue on stopped ThreadPool");
    tasks.emplace([task](){ (*task)(); });
  }
  tasks_number += 1;
  condition.notify_one();
  return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool() {
  {
    std::unique_lock<std::mutex> lock(queue_mutex);
    stop = true;
  }
  condition.notify_all();
  for (std::thread &worker : workers)
    worker.join();
}
}  // namespace xflow
#endif  // SRC_BASE_THREAD_POOL_H_
